1. Field of the Invention
This invention relates generally to the field of automatic frequency tuning in wireless applications, and more particularly to a technique for immediately and accurately tuning the local oscillator of a receiver.
2. Description of the Related Art
Wireless communications systems have become commonplace in today's society, and are used in a wide variety of applications. Such communications systems consist of several basic subassemblies, for example, a transmitter with an antenna separated by some distance from a receiver with an antenna.
A transmitter performs two basic functions. First, it generates, on the correct frequency, a signal of sufficient power to reach the area of interest. Second, it changes the “intelligence” frequency to one suitable for transmission, where intelligence may include speech, code, music, or video. To perform these functions, a transmitter must have a frequency determining circuit, or an oscillator, a modulator, and RF amplifiers.
In order to transmit intelligence, or information, the information must be raised in frequency so that it is suitable to be transmitted through free space as electromagnetic radiation. To convert information to a form suitable for transmission through free space, an electronic technique known as modulation is used. Modulation is the act of varying a characteristic associated with one wave in accordance with a characteristic associated with another wave. The base, or carrier wave, can be varied, for example, in frequency, amplitude, bandwidth, or duration of transmitter “ON” time.
Whatever type of modulation is used, the receiver in the communications system must be able to demodulate the transmitted information to extract the intelligence component. A receiver essentially takes an input consisting of modulated electromagnetic radio waves and converts these waves into a format usable by an operator. Circuitry within the receiver removes the intelligence from the RF carrier and restores it to its original frequency and format. In effect, a receiver reverses the process performed by the transmitter, through functions including, for example, reception, selection, detection, and reproduction.
Typical receivers operate by reducing the input frequency to a lower frequency for amplification, detection, and processing. The very high input signal frequency intercepted by the receiver antenna is mixed with a lower, internally generated variable frequency to produce a fixed frequency known as the intermediate frequency (IF). The intermediate frequency (IF) is detected, processed, and amplified for conversion back into the same format as the original modulating signal in the transmitter.
Typically, the input RF is mixed with a local oscillator (LO) frequency. The local oscillator (LO) attempts to control any tuning or frequency adjustments required to lock the receiver to the transmitter frequency. Automatic frequency control (AFC) circuits are used in an effort to keep the frequency stable within system parameters, because necessary adjustments often occur too rapidly for a human operator to perform. The goal of automatic frequency control (AFC) circuits is to achieve a stable intermediate frequency (IF) signal.
In other words, although receiver amplifiers typically require a predetermined frequency, wireless carrier signals tend to exhibit frequency drift. Therefore, a challenge in the field has been to convert an unstable signal to one that can be utilized by the receiver. Automatic frequency converters have traditionally provided a mechanism for tuning carrier signals so that they may be amplified by the receiver system. Typically, the local oscillator (LO) frequency is mixed with the carrier frequency in an attempt to ensure a stable intermediate signal. Often, coarse adjustments are made with a dial or channel selector. With automatic frequency tuning, directional fine adjustments may also be made iteratively using binary circuitry depending on the value of the incoming signal.
Although prior art techniques are generally good for their intended purposes, further improvements are necessary. For example, one challenge results from the common use in wireless applications of voltage-controlled oscillators (VCOs) to transmit the carrier signal. Voltage-controlled oscillators (VCOs) utilize passive components to convey the pulse, which is one reason the resulting frequency may drift about the chosen bandwidth. Often, the volume of data may be large, requiring a broader bandwidth such that even minimal drift can prevent successful transmission. As explained above, receivers often compensate for this variability by utilizing automatic frequency control (AFC) techniques. Commonly, in such techniques, the frequency is adjusted by a set of comparators that relate the analog signal to the set point of the receiver. Many algorithms make use of binary circuitry to determine whether an adjustment should be made in the positive or negative direction. The adjustment is then made iteratively until the set point is equivalent to the frequency of the incoming carrier signal. In other words, in such prior art techniques, typically the frequency is iteratively tuned according to a series of directional adjustments. There exists a great need for the frequency to be immediately tuned to the exact frequency required. The technique should ideally do so both immediately and accurately.
An example of a commonly used scheme is discussed in U.S. Pat. No. 3,946,329 to Caspari, which relates to an electronic automatic frequency tuning system. In Caspari, the frequency of the local oscillator is varied until a predetermined relationship between the local oscillator signal and a reference generator signal is identified. Then, upon identification of the predetermined relationship, the frequency of the oscillator output signal is stabilized at a substantially constant value. To stabilize the frequency of the oscillator signal, the reference generator signal and the local oscillator signal are combined and a resulting difference frequency signal is applied to a discriminator. The discriminator provides an output signal which applies a control signal to the oscillator resulting in oscillator signal frequency corrections compensating for changes in the oscillator signal frequency. The local oscillator frequency is increased or decreased accordingly. The automatic frequency tuning circuit is not dependent on the received signal, i.e., the signal for which demodulation is typically desired, and therefore a loss of the received signal does not affect the frequency of the oscillator signal. However, in the presence of the received signal, an automatic frequency control (AFC) circuit utilizing the received signal controls the frequency of the local oscillator output signal.
Prior art techniques such as those discussed in Caspari which rely for example on iterative adjustments in the positive or negative direction suffer from certain drawbacks. First, while such iterative processes may attempt to adjust the local oscillator appropriately, the exact intermediate frequency may never be achieved. In addition, as explained above, the volume of data is often large, which requires a broader bandwidth such that even minimal drift can prevent successful transmission.
Furthermore, transmitters typically have an intermediate period during which no data is sent, such as the period immediately following power-up. Therefore, a so-called dummy waveform or preamble component is typically sent by the transmitter to allow the receiver to adjust properly. In prior art systems using automatic frequency control (AFC) techniques such as those explained above, a prolonged preamble component is often needed for the system to adjust. This may be both time consuming and power consuming. In wireless applications, conserving power is a paramount concern, as a temporary power source such as a battery is often used. There is therefore a great need to provide an automatic frequency tuning (AFT) technique which can enable a shorter preamble component, thereby conserving power.
In the past, some efforts have concentrated on improving the stability of the voltage-controlled oscillator (VCO), on the theory that the more stable the source, the less time it may take to tune the local oscillator (LO). Unfortunately, such more stable components are often more costly as well, and may take up more occupied space. Since there are numerous cost-sensitive and miniaturized applications, such a design may not be appropriate.
There exists, therefore, a great need for an automatic frequency tuning system which takes an entirely fresh approach, and overcomes the above-mentioned obstacles which have heretofore plagued the prior art.